1Field of the Invention
In general, the invention relates to a method of depositing an epitaxial layer on a single crystal substrate having greater than 10% lattice mismatch therebetween and a product produced thereby. More particularly, the invention relates to epitaxial growth of TiN on (100) silicon growth of TiN on (100) GaAs.
2. Description Of Related Art
Titanium nitride films and coatings having polycrystalline microstructure have found applications ranging from corrosion and erosion resistance coatings to diffusion barriers in advanced integrated circuit devices to wavelength-selective films. (M. Wittruer, B. Studer, and H. Mechiar, J. Appl. Phys., 52, 5772 (1981); B. Zega, M. Kornmann, and J. Amiquet, Thin Solid Films, 54, 577, (1977); and E. Vulkonen, T. Karlsson, B. Karlsson, and B. O. Johansson, Proceedings of SPIE 1983 International Technical Conference, 401, 41 (1983).)
Gupta et al. report that it is common to anneal wafers at 400.degree.-450.degree. C. after aluminum metalization in order to reduce contact resistance by dissolving native oxide on silicon, reduce metal defects and minimize interface state densities in MOS devices. (G. Gupta, J. S. Song and V. Ramachandran, "Materials for Contacts, Barriers and Interconnects", Semiconductor International, October 1989, 80-87.) In order to prevent problems with respect to diffusion of Si into the aluminum during such an anneal, a metal silicide layer is interposed between Al and Si in contacts. Silicides are usually formed by depositing a metal layer such as Pt, Pd, Co, Ni, Ti, Mo, W or Ta and reacting the metal with the Si in the contact windows to form a silicide. The noble and near-noble metal silicides such as PtSi, Pd.sub.2 Si, CoSi.sub.2 and NiSi are used for devices with junction depths of at least 0.5 .mu.m if post-aluminum deposition processing temperatures do not exceed 400.degree. C. and are almost always used with a barrier. The barrier is used to prevent diffusion of Si into Al. TiN is considered an effective stable barrier used in silicon technology.
In bipolar and MOS integrated circuits wherein Al is used for interconnections, barrier layers of TiN have been proposed as diffusion barriers to prevent low temperature interdiffusion of aluminum and silicon during contact sintering, passivation or device packaging. (M. Wittmer, "Properties and microelectronic applications of thin films of refractory metal nitrides", J. Vac. Sci. Technol. A, Vol. 3, No. 4, 1797-1803, July/August 1985.). Wittmer reports that the resistivity of such TiN layers is 20-70 .mu..OMEGA.cm and that a contact structure consists of a PtSi layer for ohmic contact to silicon, a TiN diffusion barrier layer on the PtSi layer and an aluminum top layer. Although aluminum melts at around 700.degree. C., the TiN layer prevents interdiffusion after heat treatment at 600.degree. C. for 30 min. Wittmer reports that another multilayer contact consists of TiSi.sub.2 on Si, TiN on the TiSi.sub.2 layer and Al on the TiN layer, such contacts remaining stable during heat treatments up to 550.degree. C. Wittmer further reports that TiN on lightly n-doped Si forms a Schottky diode with a low barrier height of 0.49 V and TiN can also be used to form a low barrier diode on p-Si. In addition, TiN can be used as gate materials in MOS capacitors and transistors if the TiN is protected by a layer of poly-Si or SiO.sub.2 to prevent oxidation of the TiN. In this case, if the poly-Si is doped it can serve as dopant source for the diffusion of source and drain regions of MOS transistors. For instance, TiN gate p-channel MOS transistors exhibit comparable performance to Al-polycrystalline Si gate MOS transistors.
In previous studies, polycrystalline TiN films were deposited by a variety of techniques: chemical vapor deposition, ion plating, activated reactive evaporation, dc, rf and magnetron sputtering. (M. Wittruer, Appl. Phys, Lett., 36, 456 (1980); and N. W. Cheung, H. von Seefeld, M. A. Nicolet, F. Ho, and P. Iles, J. Appl: Phys., 52, 4297 (1981).) The polycrystalline films tend to grow columnar with grain boundaries normal to the substrate. In the columnar structure, faster diffusion along the grain boundaries makes these films susceptible to environmental degradation and reduces their effectiveness as a diffusion barrier. The above problems in polycrystalline films can be solved by producing an equiaxed microstructure, where grain boundaries are randomized and direct fast diffusion paths between the interface and the free surface are eliminated.
Hultman et al. report growing single-crystal TiN films on MgO substrates by magnetron sputtering. (L. Hultman, G. Hakansson, U. Wahlstrom, J. E. Sundgren, I. Petrov, F. Adibi and J. E. Greene, Thin Solid Films, 205 (1991), 153- 164.) Hultman et al. report that TiN grown on steel had a polycrystalline columnar structure with open grain boundaries and faceted surfaces.
Polycrystalline TiN films with equiaxed-grain microstructure have been produced by laser physical vapor deposition (LPVD) in the temperature range 400.degree.-500.degree. C. on silicon substrates compared to higher temperatures (in excess of 900.degree. C.) needed for chemical vapor deposition methods. (N. Biunno, J. Narayan, S. K. Hofmeister, A. R. Srivatsa, and R. K. Singh, Appl, Phys. Lett., 54, 1519 (1989).) In addition, single crystal TiN films have been formed by LPVD on lattice-matched substrates such as MgO(100) in the temperature range 450.degree. to 750.degree. C. (N. Biunno, J. Narayan, A. R. Srivatsa, and 0. W. Holland, Appl, Phys. Lett., 55,405 (1989).) The lattice mismatch between the TiN film and the MgO substrate is 1% and resistivity of the TiN was around 50 .mu..OMEGA.cm or higher. Epitaxial growth of TiN thin films on (100) Si is reported by Choi et al but in actuality, the TiN film is textured since it only has one axis aligned. (C. H. Choi, L. Hultman, W. A. Chiou and S. A. Barnett, J. Vac,Sci, Technol. B, Vol. 9, No. 2, March/April 1991, 221-227.) Epitaxial films, however, have all three axes aligned.
Johansson et al. reported growth of single crystal TiN films on (111)MgO in the temperature range 600.degree.-800.degree. C. using a reactive magnetron sputtering technique. (B. 0. Johansson, J. E. Sundgren, J. E. Green, A. Rochett, and S. A. Barnett, J. Vac, Sci. Technol., A3,303 (1985).) In LPVD, the epitaxial films have been obtained at a temperature as low as 450.degree. C. The higher energy of laser evaporated species .about.5-8 ev per particle compared to thermal evaporation (.about.0.1 ev per particle) is envisaged to enhance the surface mobility and provide recrystallization at a lower temperature. (For example, see Biunno et al. supra.)
There is a need in the art for a process capable of forming single crystal TiN (a=4.24 .ANG.) films on lattice-mismatched substrates such as (100) silicon (a=5.43 .ANG.) and GaAs (a=5.65 .ANG.). (L. E. Toth, Transition Metal Carbides and Nitrides (Academic, N.Y., 1971).) Such a process would be expected to revolutionize the fabrication of advanced integrated circuits. Single crystal films could eliminate fast diffusion paths associated with grain boundaries and the low-resistivity of single crystal films could make them suitable for contact metallurgy such as ohmic contacts in advanced integrated circuit devices.